autocmd setting signs is breaking syntax highlighting - vim

Question

I currently have an autocmd for BufAdd that will add marks (signs) to certain lines of the loaded file. I have defined custom highlights for each sign, along the lines of:
highlight Custom1 ctermfg=black ctermbg=red
highlight Custom2 ctermfg=black ctermbg=green
sign define custom_sign1 text=>> texthl=Custom1
sign define custom_sign2 text=<< texthl=Custom2
Signs are create with:
sign place ID line=LINE name=custom_sign1 file=FILENAME
where ID is a unique id for the sign in the file, LINE is the line number for the sign, and FILENAME is the name of the file.
The signs all get created correctly, but it kills the syntax highlighting. I then have to do syntax on again, but then the custom colors for the signs don't show, at which point I have to reload the vimscript that defines those colors.
How do I make this not break syntax highlighting?
Also, this is done from an autocmd that uses the BufAdd and VimEnter events.
Edit: wrote the wrong BufXXX event at the end of the post - should be BufAdd
Update:
To duplicate my problem, source the .vim file below, and then open a new buffer that is normally syntax-highlighted. The first line will have a red >> sign, but the file will not be syntax highlighted. Note that it must be a new buffer (not already in the buffer list):
highlight Custom1 ctermfg=black ctermbg=red
highlight Custom2 ctermfg=black ctermbg=green
sign define custom_sign1 text=>> texthl=Custom1
sign define custom_sign2 text=<< texthl=Custom2
function! SignLine()
execute "sign place 1 line=1 name=custom_sign1 file=" . expand("<afile>")
endfunction
autocmd!
autocmd BufAdd * call SignLine()
augroup END
I created a simple file called test.c. After opening vim with the above .vim script as the initial file, sourcing it (with :so %), and then opening test.c, this is what I see:
After turning the syntax back on with :syntax on, it now looks like this (custom sign colors are now broken):
To get the custom sign colors working again, I have to re-source the vimscript AGAIN (:so test.vim), after which both the custom signs and the syntax highlighting work:
I shouldn't have to do this. Why is this happening?

Answer 1

When opening your script in Vim, the syntax highlighting already gives a big clue:
That last line isn't highlighted correctly because there's no :augroup command that corresponds to augroup END! You need to define it like that (or combine the two inner lines to autocmd! BufAdd ...):
augroup SignLine
autocmd!
autocmd BufAdd * call SignLine()
augroup END
The initial :autocmd! removed all defined autocmds, also the ones responsible for loading the syntax. That explains the broken behavior.